This invention relates to a vacuum circuit breaker, and more particularly to one working at rated voltage of 3.6 to 36 KV and rated breaking current of 8 to 60 KA.
There is a chopping phenomenon, which is a phenomenon particular to the vacuum circuit breaker. The phenomenon is one in which a current chops suddenly before it comes down naturally to a zero point at the time of breaking a circuit or, particularly, a small current. The current at the time of such chopping occurring is called the chopping current. An occurrence of chopping may lead to an abnormally high surge voltage on equipment at the load side such as a rotary machine and transformer, with the result that dielectric breakdown is apt to occur. The larger the value of the chopping current, the more that dielectric breakdown becomes apt to occur.
On the other hand, in a vacuum circuit breaker there flows not only a rated current but occasionally there also flows a short-circuit current that is far larger than the rated current. Even in such case, it is necessary for the vacuum circuit breaker to operate normally so as to break the short-circuit current. It is therefore desirable that the vacuum circuit breaker has the characteristic of small chopping current for making the surge voltage small and a breaking a large current. The matter that the breaker is capable of breaking large currents is hereinafter referred to as "breaking performance". The better the breaking performance is, i.e., the larger the current value capable of being broken is, the more the vacuum circuit breaker becomes capable of effecting the breaking in a case of a short-circuit accident, thus the safety of the vacuum circuit breaker being improved.
To improve the chopping current and breaking performances, there have been hitherto effected many attempts mainly to improve the material of the electrodes. For example, in U.S. Pat. No. 3,014,110, U.S. Pat. No. 3,683,138 and U.S. Pat. No. 3,993,481 specifications, there are shown examples in which electrode materials are improved in view of chopping current. In the specification of U.S. Pat. No. 3,683,138 there is shown a contact made of a sintered alloy of Ag and WC; the specification of U.S. Pat. No. 3,993,481 discloses a contact made of another alloy in which there are dispersed T, Bi, Pb and etc. in a matrix of an eutectic alloy including Co and other elements. Generally, however, the situation is such that the vacuum circuit breaker with small chopping current characteristic is inferior in breaking performance, while in other vacuum circuit breakers with superior breaking performance, the chopping current becomes large in value.